The present invention relates to a communications system in which access to a communications medium used by a plurality of nodes in a shared fashion is suitably controlled. The invention additionally relates to methods of controlling access to a communications medium.
In many communications systems a plurality of nodes can communicate with one another via a shared communications medium. In this case, it must be ensured that different users can be separated. If a plurality of users transmit data simultaneously in an uncontrolled manner, meaningful detection of individual users may not be possible. Therefore, control of access to the shared medium is an important element in communications technology.
In so-called “competing methods” the various nodes of the system are in competition for access to the shared communications medium. Examples of such methods include carrier sense multiple access (CSMA), ALOHA and Slotted ALOHA.
With the use of this method, collisions can occur, however, in the event of different subscribers attempting access simultaneously. Particularly in systems having a multiplicity of subscribers and/or a high level of traffic, this can result in a lower (accumulated) data throughput. Moreover, hard real-time requirements can be guaranteed only with great difficulty or cannot be guaranteed at all. If the subscribers are supplied with energy via the shared communications medium itself, simultaneous attempts at access by different subscribers can furthermore be problematic from this standpoint as well. This can be the case particularly if the maximum power consumption is restricted, for example for safety reasons, since the power consumption in the sending mode is usually higher than that in the receiving mode.
By contrast, collisions can be avoided with use of so-called “coordinated methods” in the error-free case. This involves carrying out a defined, exclusive allocation of different resources to different subscribers. These can be time slots, frequency ranges, codes in a code division multiplex method, or the like. The coordination is usually effected here either by a central entity (as in the case of traditional polling or scheduling methods), with the aid of a distributed approach (as is the case for example for “token ring” or “token bus”) or by means of a static configuration (as in the case of static time division multiplex methods, for example).
One typical example of a coordinated media access method is a traditional polling approach, in which a central entity as coordinator cyclically interrogates the different subscribers of the system as to whether they have data to transmit. If appropriate, the central entity then temporarily grants access to the shared communications medium.
When a spectrally efficient method is used, which generally has a relatively high complexity (for example with regard to the coding or modulation methods used), a comparatively high energy feed is normally necessary. This is the case particularly at the receiving end, where said method also has the greatest effect because, in general, the number of receiving subscribers or subscribers ready to receive is very much greater than the number of sending subscribers.
Such a high energy feed is often undesirable, however. It can typically be avoided, however, only by the use of a relatively simple, but in return inefficient transmission method. Particularly in the case of embedded communications systems and sensor networks in which only very limited resources are available, endeavors are made to lower energy costs and to increase the maximum possible operating period of battery-operated devices.
Minimizing the energy requirement is additionally of importance if the various nodes are supplied with energy via the shared communications medium itself. A lower energy requirement of an individual node then allows correspondingly more nodes to be connectable to the system, which ultimately in general again leads to lower overall costs.
In accordance with known communications systems, therefore, it is necessary to make a compromise between accumulated power consumption and achievable performance. If a complex, sophisticated transmission method is used, it may be possible to obtain high data rates and low latencies, but in return the overall power consumption is relatively high because all the subscribers must always attempt to detect corresponding signals.